Process of making thiocyanates



Patented 20,

ra'ocsss or 'George'ltiethot,.Mount Lebanon, Pa., assignor to PittsburghCoke & 'Iron Company, Pittsburgh, 2a., a corporation of Pennsylvania NoDrawing.

Application December Serial No. 512,969

6 Claims. (cl. at -7 5) 'rne presentinvention relates to themanui'acture of thiocyanates', and particularly alkali met a1 .thiocyanates,such as those oi. sodium and potassium. I I a The primary object of theinvention is to form thiocyanates by the reaction or a soluble cyanidewithsulphur in the'prsence of a solventfor sulphur,-such as ammonium andalkali metal sol-- Phides.-1 V

Anotherand equally important object 01' the invention is to carry outthe reaction using an amount or sulphide which is less than the equi 7molecular amount of alkali metal cyanide.

I have discovered that alkali metal thiocyanates can be produced by thereaction. of sulphur with asoluble cyanide in the presence of a'sulphidewhich is a solvent for the sulphur and in a manner which is equally aseflicient as the wellknown reaction between a-soluble polysulphide, suchas ammonium polysulphide, and a soluble cyanide to produce ammoniumthiocyanate. The latter process is used on a large scale and presents-nodiillculties since the ammonia,-hydrogen sulphide and hydrocyanic acidformed are easily removable. However, when it was attemptout initiallyat room temperature and as the reaction proceeds, the temperature risesbut whenthe. dilution becomes greater than-the heat oi."

reaction, 'which is exothermic, is not suflicient to'-"prom ote enoughrapid reaction. Therefore, 7

some .heat hasto be' applied but care must be taken that thetemperatureis maintained below the point at which sulphur becomes plastic or lumps..The maximum temperature is in the neighborhood of 100 C. This additionis made continuously in amount sumcient for the cyanide to react withthe sulphur, which is continuously dissolved as the reaction continuesuntil all of ed to follow thesame procedure with alkali metalpolysulphides, difil'culties arose in removing the sulphide produced.For instance, in the producthe cyanide has been converted to thethiocy-, anate. 'The alkali metal and ammonium sulphides dissolvesulphur in exceedingly dilute solutions at elevated temperatures, andonly a rela-- tively small amount of sulphide is necessary, 1. e.,substantially less than equimolecular proportions based on the amount ofsoluble cyanide in order q to convert a large amount of sodium cyanideinto sodium thiocyanate.

I Thus, in one experiment the ratio of ammo nium sulphide tosulphur-was. 1:140. This means thatonly 6 grams of ammonium sulphide, or7 grams of sodium sulphide, were necessary to produce 1000 grams ofsodium thiocyanate. Such small amount of ammonium oralkali sulphide tionof sodium thiocyanate b the known procetime, an equivalent amount ofsodium sulphide always remained in solution according to the equation:NazSz-i-NaCN=NaCNS,-i-Na2S. The

solution contained96 grams of Nags for each 100- grams of sodiumthiocyanate produced. This condition remained notwithstanding the use ofthe highest polysulphide, for according to the reaction: NazSs+4NaCN=4NaCNS +N'azS there was obtained 24 grams of sodium sulphidefor each 100 grams of sodium thiocyanate. In other words, equi-molecularproportions of the alkali metalcyanide and the sulphide were requiredand upon completion of the reaction it was found that such a largeamount of sulphide produced remained in the solution that it could beremoved only with. diillculty. "This made the process ineflicient andcommercially unattractive.

According to the present invention, alkali nietal 1 and ammoniumsulphides are employed to dissolve the-sulphur for reaction with thesoluble Y cyanide, and in substantially less than equi-molecular amountwith respect to the c ani de. 'I'hecyanide in aqueous solution is dedslowly to the aqueous solution of sulphide containing dissolved sulphurwhereby the cyanide reacts with the sulphurin solution' The reaction iscarried used is economically of noimportance and can be boiled oil inthe case of ammoni m sulphide or converted to alkali carbonate; by meansof CO2 in the' case of alkali sulphide. In the projduction of sodiumthiocyanate the sodium carbonate formed is only slightly soluble inconcentrated sodium thiocyanate solutions and can therefore beremovedbyfiltration.

' Emmple 400 grams of technical sulphur, for instance as purchased forthe production of sulphuric acid,

are ground to less than 20 mesh and put ina 3 liter flask fitted with anair condenser. To this is added 100 cc. of an aqueous solutioncontaining approximately 20% ammoniumsulphide. This is a molecularratio'of ammonium sulphide to sulphur of 1:42. To this mixture is addedslowly and with agitation about 1700 cc. of an aqueous solution ofapproximately 30% sodium cyanide. Care-should be taken that sulphur isalways in solution as can be readily determined by the color of thesolution, and also that the temperature does not become 'too highthereby causing loss of ammonium sulphide or lumping of the sulphur.when about hall of the sodium cyanide solution has added, theflask-isheated to a maximum of 90 C. and the addition 4 of the cyanide solutionis continued. After all the sulphur has gone into solution, a slightexcess of sodium cyanide is added to react with and thereby remove alldissolved sulphur, the air condenser is removed and (302 at elevatedtemperature is bubbled through the solution, which is kept constantlyboiling untilall hydrogen sulphide is removed and any excess of sothesolution is cooled below 100 C. and then filtered from the impurities oithe sulphur and the precipitated sodium carbonate.

For the production of sodium thiocyanate crystals the filtrate isevaporated further preferably to" a boiling point of 145-150 C. and thecrystals are recovered in the usual way.

A substantially similar procedure is employed for making other alkalimetal thiocyanates e. g. or potassium.

In the foregoing example, sodium sulphide may be employed in lieu ofammonium sulphide for the manufacture of sodium thiocyanate. vLikewise,potassium sulphide may be used in lieu of ammonium sulphide togetherwith potassium cyanide for the manufacture of potassium thiocyanat'e.

By the expression the amount or the sulphide present being less than anequi-molecular amount of the cyanidejin the claims, I mean that forevery four molecular weights of sodium cyanide using sodium sulphide, orfive molecular weights of potassium cyanide using potassium sulphide,

or eight molecular weights of alkali metal cyanide using ammoniumsulphide, there is present less than one molecular weight of ammonium oralkali metal sulphide. In other words, the total amount of cyanidereacted is appreciably in excess of the polysulfide present, wherebylarge amounts of thiocyanate are formed with an amount of polysulfidewhich is less than an equimolecular amount of the cyanide. Of course,the molecular ratio of sulphur to cyanide must be at least 01: l.

I claim:

I l. The method of preparing alkali metal thiccyanates which comprisesadding sulphur to an aqueous solution of sulphide selected from thegroup consisting of ammonium and alkali metal sulphides which dissolvesulphur and form polysulphides, reacting the polysulphides withsuccessive increments of an aqueous solution or an alkali metal cyanide,the total amount of cyanide so reacted being appreciably in excess ofthe polysulphide present, whereby large amounts of thiccyanate areformed with an amount or polysulphide which is less than anequimolecular amount of the cyanide.

2. The process in accordance with claim 1, Y

wherein the sulphide is ammonium sulphide.

3. The process in accordance with claim 1, wherein the sulphide isanalkali metal sulphide.

4. The process in accordance with claim 1, wherein the sulphur is addedto an aqueous solution of the sulphide and thereafter the cyanide isslowly added to react with the sulphur which is in solution, in thesulphide until the reaction is completed.

5. The process in accordance with claim 1, wherein the reaction iscarried out at temperatures below the temperature at which sulphurbecomes plastic.

,6. The process according the reaction is carried out at temperatures upto about 100 C.

GEORGE RIETHOF.

to claim 1, in which

